The present invention relates to an X-ray attenuation correction method for correcting for the attenuation of X-ray beam emitted to a subject, an image generating apparatus, an X-ray CT apparatus, and an image generating method.
The X-ray CT apparatuses have been widely used these days, and the image quality requirement for tomographic image information obtained by an X-ray CT apparatus is continuously becoming finer. There are methods for improving the image quality of the X-ray CT images, which include for example the correction of the dispersed X-ray generated within a subject.
In the above method the amount of scattered X-ray theoretically or experimentally determined is eliminated from the projection information of the subject. The projection information which is the basis of the image reconstruction is configured so as to be comprised of solely the transmitted X-ray beam.
The amount of scattered X-ray from the subject increases in relation to the transmission distance of the X-ray beam passing through the subject (referred to as projection length herein below). Therefore the projection length should be considered when correcting for the scattered X-ray, so that the amount of scattered X-ray to be corrected for must be larger when the projection length is longer, namely, the subject has a larger thickness in the direction of the projection.
However, in accordance with the background art described above, the correction of the scattered X-ray based on the projection length is not sufficient. More specifically, when there is a boundary changing the absorption rate of X-ray on the path of the X-ray beam transmission, the X-ray beam will be attenuated on the boundary. The attenuation is a phenomenon different from the dispersion of X-ray beam seen in a uniform medium, which cannot be corrected for by considering the projection length.
The attenuation of the X-ray beam on the boundary, in particular, may generate artifacts on an image in vicinity of the boundary where the X-ray absorption rate is changing, during the formation of a tomographic image by device of image reconstruction. The artifact may be a cause of degrading the image quality of tomographic image, thus this is never preferable.
Therefore, it is important to achieve an X-ray attenuation correction method for correcting for the X-ray beam attenuation on the boundary where the X-ray absorption rate of the subject is changing, and an X-ray CT apparatus.